8 Answers
8

Dowels and biscuits are length-preserving, cheap and common ways of generating some long-grain glueing surface. They're not quite as strong as other solutions, but usually entirely sufficient.
For dowels, you only need a drill and transfer plugs (or a dowel jig, which is more expensive). If you don't even have transfer plugs, you can improvise them with a dowel and a nail, or even with a dowel and a felt marker (no joke, done that before...). For biscuits, there is a special kind of tool, the "biscuit joiner", which makes sawing the slits in an automated process (not exactly the Mount Everest of traditional craftmanship, but it works very well).

Tongue and groove is very slightly more work than the above. If you have appropriate tools (a router will for example do, but manual tools are perfectly fine), it's a matter of minutes. I'm using a plow plane for that since taking the router out of the box and assembling/adjusting everything takes about as long as it will take me doing the tongue and groove with the plow plane, and I don't need hearing protection, nor is there a lot of fine dust. Hand saw and chisel certainly works, too, for the patient and dedicated traditional craftsman.
The tongue can also be loose (a separate piece of wood) which is basically like a single large biscuit going all the way through. The advantage of a loose tongue is that you do not lose length. Mind that the external appearance is different, you'll see end-grain at the tongue, which can be undesirable but can also be very desirable (indeed sometimes people deliberately use a completely different wood for contrast, it can look pretty cool).

Then of course there are about two dozen variations of joinery that you could use, all of which do lose some length, however.
What to use is hard to answer, as it depends on how much work you want to put into this, how stable it has to be, in which direction the expected force will act, and what "look" you want.

A simple lap joint, for example, is perfectly fine if you do not have any "special needs" on stability. It can be made easily and quickly with no tools but saw and chisel, or with a rabbet plane (or, of course, if you have a router, with the router).

There is also a bit for routers that can make what is called a finger joint. It makes many narrow interlocking 'fingers' to increase the gluing surface area. This was designed for joining boards end to end.

You can use a long lap joint here, but one of the standard ways to join two boards like this when cutting by hand is to form what's called a scarf joint. This is approved by the Federal Aviation Administration so if it is strong enough for airplanes it probably will suffice for your use as well. The key for FAA approval is 12x to 15x angle measured as length/thickness as shown in the following diagram as (8x):

(note the detail of the length of the cut face). This cuts off the end grain and creates two long-grain faces that then glue up much more strongly. However, you may not end up with a piece of wood as strong as a complete board unless you use further reinforcement.

There are other forms of scarf joint, some with interlocking features that look like what we now associate with Japanese joinery, and while they create joins that are much stronger in many cases they are far more challenging to cut accurately. See more in the Scarf Joint section of William Fairham's classic book, Woodwork Joints which you can find on Project Gutenberg.

The scarf joint is the common solution used by boat builders for joining end to end. I also recommend Fairham's book (good illustrations and it is still in print)!
– ewmApr 2 '15 at 17:03

How do you cut that scarf joint? Looks challenging.
– Norman RamseyJul 14 '18 at 15:51

@NormanRamsey, depends on what tools you have at your disposal. Quite easy to do this with a table saw with the right jig for example. But obviously traditionally it would have been sawn by hand using whatever suitable saw was in use in that place and time. But "cutting" the joint may not give the right impression any more, in the past it would almost certainly have been glued straight from the saw, these days where most modern glues require very smooth surfaces for maximum strength it would be better to carefully pare or plane the joint perfectly flat and smooth after sawing.
– GraphusJul 14 '18 at 18:45

I've seen traditional scarf joints described variously using a planer and a planer sled.
– jdvMar 11 at 15:38

You could make a lap joint with a router, then put a few dowels of appropriate size through the joint at right angles to the boards to tie everything together so you're not counting on the shear strength of the glue to hold it all together. IMO this is much easier to put together than a scarf joint as long as appearance is not critical, and as long as you don't mind dragging out the router

In the woodworking industry, we have this type of joint very often on countertops, and other field joint scenarios. We usually use a combination of biscuits, for alignment, and tite joint fasteners, to pull the two pieces together.

If you really want strength, you could cut a Kanawa Tsugi! It's a keyed scarf joint taken from the timber framing tradition of Japanese temple builders. There are examples where this joint has held up for thousands of years.

Cut both of your boards so that they have the following profile.

Apart from the slanted cuts in the middle, all angles here are square. The slant I used here is 5 degrees against the straight edge of the board, but the exact angles and measurements aren't really important, so long as you make sure to use the same for both boards. Flip them and line them up while marking, then flip them back to face each other after the cuts.

Now line up the straight edges of the tips and bring the profiles together.

Since they were cut symmetrically, they should fit together snugly.

Now push them together end to end.

The slanted surfaces will slide against each other and guide together the mini-lap joints at the ends.

It will be hard to push them all the way in, but once they are over the lip, the joint is ready for the final step. Fit a square dowel into the hole in the middle, then pound it in with a mallet, closing the remaining gaps.

The final joint should be stronger than both boards were to begin with, and nearly impervious to wood movement, as expansion along the grain only serves to increase tension within the joint.